1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation, and in particular to such an installation suitable for subtraction angiography.
2. Description of the Prior Art
X-ray systems used in subtraction angiography generally include an x-ray radiator which generates an x-ray beam that penetrates a patient, with the resulting x-ray image being processed in an x-ray image intensifier video chain, wherein the x-ray image is converted into a sequence of video signals. The video chain includes an image memory in which video signals are stored, and a subtraction unit wherein the stored video signals are subtracted from current incoming video signals so that a difference video signal is formed. A playback or reproduction system is provided for displaying the difference video signals, which includes controls for varying parameters which identify a region or a window of brightness values in the display of the difference video signals on a monitor.
Such installations are employed in digital subtraction angiography (DSA) which enables a radiological portrayal of blood vessels without the superimposition of bones, air and soft tissue thereon. A contrast agent is for this purpose injected in a blood vessel at the beginning of an exposure series. A "dummy" exposure is made for use as a mask, before the contrast agent appears in the vessels of the region under examination. The stored dummy image is subtracted from the "filled" image in a filling phase following immediately thereafter, wherein the concentration of the contrast agent in the vessels gradually increases, the concentration rising to a maximum and then decaying. Only image structures containing the contrast agent are obtained. The difference image displayed on the monitor usually has a low contrast. For this reason, it is intensified or "windowed."
A gray scale region-of-interest of the difference image can be displayed over the full brightness range of the image monitor as a result of the windowing. Gray scale values of the difference image which lie outside the window are shown only as black or white on the image monitor.
An apparatus for windowing is disclosed in U.S. Pat. No. 4,827,492, wherein two operating elements are provided for setting the window. The window width is set with one operating element and the window center and the upper or lower window boundary are set with the other operating element. A disadvantage of this known apparatus is that the background brightness of the subtraction image and the darkening (optical density) of the blood vessels cannot be set independently of each other. The user is therefore forced to undertake an optimization of the image in alternation using both operating elements. Reproducible image results, however, can be achieved only with difficulty in this way. The maintenance of reproducible image results is required, for example, for a hard copy documentation which must be kept constant in quality, such as on a film.